1. Field of the Invention
The present invention relates to the structure of a mask for X-ray lithography (hereinafter referred to as an X-ray mask) serving as an obturating member in the formation of fine patterns of not more than 1 .mu.m in size, i.e., in the submicron range, such as circuit patterns employed for fabricating a very large scale integrated circuit device (VLSI) through a known X-ray exposure technique (X-ray lithography).
2. Description of the Prior Art
FIG. 1 is a sectional view showing a conventional X-ray mask, which is disclosed in "X-Ray Lithography" by R. K. Watts, May 1979/Solid State Technology, for example. Referring to FIG. 1, a transparent thin film 1, being permeable to visible rays and X-rays, is formed by an insulator of 2 to 3 .mu.m thickness of a material which is excellent in strength, such as BN, SiN or SiC. X-ray absorbers 2 of heavy metals having high X-ray absorptivity, such as Au, W and Ta, are formed in lamination on the surface of the transparent thin film 1, in correspondence to patterns to be created. The aforementioned literature illustrates an X-ray mask which comprises a transparent thin film 1 of SiC and X-ray absorbers 2 of a Ti film of 150 .ANG. thickness, an Au film of 5000 .ANG. thickness and a V film 800 .ANG. in thickness, which are formed on the SiC thin film. A ring-shaped support member 3 is provided along the peripheral edge portion of the back surface of the transparent thin film 1, to support the transparent thin film 1 and the X-ray absorbers 2. An X-ray transmission window 4, which is an opening defined in the support member 3, has a region corresponding in size to the patterns to be created.
In order to perform exposure through the X-ray mask of such structure, the X-ray mask is first superposed and positioned with a substrate to be exposed, e.g., a semiconductor substrate by visible rays. Visible rays are perpendicularly applied to the surface of the X-ray mask to permeate the same, so that the operation for superposing and positioning the X-ray mask with the substrate is performed on the basis of light visibly reflected by the substrate. Therefore, the transparent thin film 1 must be permeable to visible rays as well as to X-rays. Upon completion of such positioning, X-rays are perpendicularly applied to the surface of the substrate from above, so that those striking the X-ray absorbers 2 are absorbed by the same while those striking other regions permeate the transparent thin film 1 to enter the substrate in correspondence to patterns which are in reverse to the patterns of the X-ray absorbers 2. Thus, as noted the transparent thin film 1 must be permeable both to X-rays and to visible light.
Since X-rays thus enter the substrate in patterns reverse to the patterns of the X-ray absorbers 2, the quality of the patterns formed in the substrate depends on the quality of the X-ray mask patterns. Therefore, the patterns of the X-ray mask must be inspected in advance of employment.
However, the minimum line width of patterns on an X-ray mask is generally less than about 0.5 .mu.m, and hence sufficient resolution cannot be obtained in pattern inspection through conventional optical techniques, whereby defects, etc., may be overlooked or not detected. Thus, pattern inspection of an X-ray mask has been performed through a pattern inspection apparatus utilizing an electron beam, as disclosed, for example, in Japanese Patent Laying-Open Gazette No. 200415/1986, No. 40146/1987 etc.
The conventional X-ray mask of the aforementioned structure is subjected to pattern inspection by the pattern inspection apparatus utilizing electron beam. Thus, when an electron beam is applied to the transparent thin film 1 a in pattern inspection, the transparent thin film 1 is electrified by injection of electrons, whereby the subsequently applied electron beam is deflected by such electrification of the transparent thin film 1. Consequently, the subsequent electron beam is deflected from its target position, whereby correct pattern inspection cannot be performed.
FIG. 2 shows another conventional X-ray mask, which has been developed to prevent such electrification of the transparent thin film 1, as disclosed in "X-ray lithography: fabrication of masks and very large scale integrated devices", SPIE Vol. 333, Submicron Lithography (1982) and "Defect Repair Techniques for X-ray Masks", SPIE Vol. 471, Electron-Beam, X-Ray, and Ion-Beam Techniques for Submicron-meter Lithographies III (1984). The X-ray mask as shown in FIG. 2 is different from that of FIG. 1 in that a transparent conductive thin film 5 is provided between a transparent thin film 1 and X-ray absorbers 2. According to the first of these references, for example, a Ti film of 10000 .ANG. thickness is formed as transparent conductive thin film 5 on a polyimide film of 16000 .ANG. thickness serving as the transparent thin film 1, and an Au film of 7000 .ANG. thickness and a TaO.sub.x film of 1400 .ANG. thickness are formed thereon as the X-ray absorbers 2. According to the latter reference, on the other hand, a polyimide film of 2 .mu.m thickness is formed on a BN film of 4.5 .mu.m thickness serving as the transparent thin film 1, and a Ta film of 300 .ANG. thickness is formed thereon as the transparent conductive film 5, while an Au film of 6500 .ANG. thickness and a Ta film of 800 .ANG. thickness are formed as the X-ray absorbers 2. The latter reference mentions that electrification is caused although X-ray mask patterns as created are subjected to pattern correction by focusing of an ion beam. The transparent conductive thin film 5 is permeable to visible rays and X-rays, similarly to the transparent thin film 1. In this X-ray mask, electrons charged in the transparent thin film 1 escape to the ground through the transparent conductive thin film 5.
In case of thus sandwiching the transparent conductive thin film 5 between the transparent thin film 1 and the X-ray absorbers 2, materials and film forming conditions must be determined consideration of differences in their adhesive properties and the differences in expansibility between the transparent thin film 1, the X-ray absorbers 2 and the transparent conductive thin film 5 in order to manufacture the X-ray mask. The manufacturing steps in such a process are complicated, with a requirement for strict manufacturing conditions.